1. Field of the Invention
The present invention relates to a honeycomb structure which may serve, for example, as a filter for collecting and removing particulates etc. in exhaust gas discharged from an internal combustion engine such as, for example, a diesel engine.
2. Description of the Related Art
Recently, it has been a problem that particulates such as soot contained in exhaust gas discharged from internal combustion engines of vehicles such as, for example, engines in buses or trucks, construction machines, etc. are harmful to environment or human bodies.
Therefore, there have been proposed various filters using honeycomb structures made of porous ceramics for collecting particulates in exhaust gas and purifying the exhaust gas.
As such a honeycomb structure in the background art, there has been disclosed a honeycomb filter in which a plurality of cells are defined by cell walls, and fluid including particulates is purified by the cell walls. For example, the honeycomb structure is characterized in that the specific surface area of particles forming the cell walls is not lower than 0.1 m2/g (for example, see Patent Document 1).
In such a honeycomb structure, a loss in pressure is small because the specific surface area of the particles forming the cell walls is not lower than 0.1 m2/g. In addition, Patent Document 1 discloses that the honeycomb structure can prevent the lowering of efficiency in collecting particulates when the specific surface area of the particles forming the cell walls is not higher than 1.0 m2/g.    [Patent Document 1] JP 2001-96112 A
The entire disclosure of JP 2001-96112 A is hereby incorporated by reference.
Generally, a filter serving to collect particulates in exhaust gas and convert the exhaust gas from gas components such as CO, HC, etc. by means of a catalyst is requested to have a high exhaust gas purifying (converting) rate at a comparatively early stage after the start of an engine. To this end, it has been believed that the heat capacity of the honeycomb structure has to be reduced.
Means for providing a high porosity to the honeycomb structure and means for reducing the thickness of the cell walls of the honeycomb structure have been proposed as means for reducing the heat capacity of the honeycomb structure.
However, if the porosity of the honeycomb structure is increased or the cell walls are thinned, particulates will pass the cell walls easily so that the efficiency in collecting the particulates may deteriorate.
Even when the specific surface area of the particles forming the cell walls has a predetermined value as in the honeycomb structure disclosed in Patent Document 1, there is no consideration about the mutual binding condition of the particles forming the cell walls, the number of particles per unit volume, etc. Therefore, it may be impossible to ensure sufficient efficiency in collecting the particulates when the cell walls have a wide variety of thicknesses.